Fluorine removal from phosphate liquors



Patented Feb. 23, 1943 FLUORINE REMOVAL FROM PHOSPHATE I LIQUORS Sihon C. Ogburn, Jr., Westiield, N. J assignor to General Chemical Company, New York, N. Y., a

corporation of New York No Drawing. Application March 18, 1940,

- Serial No. 324,051

iClaims. (01.23-109) This invention relates to treatment of phosphate liquors to remove fluorine therefrom, and more particularly to the removal of small amounts of fluorine impurities from phosphate liquors, especially monocalcium phosphate liquors, to produce food grade phosphates. The term liquors is used herein to include solutions and slurries of phosphates.

Government regulation in connection with, and purchasers specifications for, food grade phosphate products rigidly limit the amount of fluorine permitted therein; at the present time monocalcium phosphate containing more than .001% fluorine is considered unsatisfactory for food purposes. While the production of monocalcium phosphate containing less than .00l% fluorine is feasible by reacting high grade hydrated lime with food grade phosphoric acid produced from blast or electric furnace operation as is well known in the art, the production of suchfood grade monocalcium phosphate from crude phosphoric. acid made-by reacting phosphate rock with sulfuric acid has presented substantial difflculties. As contrasted with food grade phosphoric acid, which usually contains about .0003% fluorine, crude phosphoric acid made from phosphate rock, even after known treatments, such as blowing with air or steam to reduce the fluorine content, contains about .003% fluorine. In the processing of such phosphoric acid by heretofore known methods to produce monocalcium phosphate or other phosphate salts, phosphate products produced invariably contained from ,.003% to .03% fluorine.

It is an object of this invention tofprovide a process for the treatment of phosphate liquors high fluorine content, say in excess of about .03%,

froman economic standpoint it will be found desirable to treat liquors having such high fluorine content by blowing with air,steam or other known process, to reduce the fluorine content to:

a value not exceeding about .03% and then treat the liquors with bone black-in accordance with this invention to remove fluorine to a point suchthat a food grade phosphate product can be obtained. In the treatment of liquor having a in such procedure or.the use of equipment in addition to that normally employed in such procedure. Other objects and" advantages of this invention will appear from the following detailed description thereof.

In accordance with this invention, phosphate liquors containing free acidity in amount not exceeding 3%, expressed as P20 are treated range of about 1.5 to 3.5." The use of bone black in accordance with this with a small amount of bone black, the amount used being proportional to the amount of flufree acidity exceeding about 3% expressed as free P205, the free acid should be neutralized or otherwise reduced before the bone black treat? ment to remove fluorine impurities.

Bone black, as is well known, is produced by the destructive distillation of bones in the absence of air, and is constituted chiefly o'f Ca3(PO4)2, and contains a minor amount (of the order of 11%) of oxidlzable material, which is mostly carbon in ahtivated .form. The bone black used may be powdered or granular; granular material of a state of subdivision such that it passes through a 50.1nesh screen but is retained on an mesh screenhas been found satisfactory. The use of finer material, however, has

'the advantage that it will act more quickly'in veffecting fluorine removal. While the explanation for the mechanismof the reaction between the bone black and fluorine present in phosphate. liquors is not definitely known, it is believed that the reaction is probably both chemical and physico-chemical, e. g. adsorption. In the case of the treatment of calcium phosphate liquors, it is believed complex calcium-fluoro-phosphates are formed which are extremely insoluble in solutions having a pH value equal to that of the solutions treated in accordance with this invention, '1. e. solutions having, at room temperature, a

invention, it has been found; results in-a phosphate product of improved color; the bone black effects or aids in effecting removal of colorpH with inthe forming bodies. Also, it functions as a filtering aid to effect a more efllcient separation of the phosp ate liquor from the impurities precipitated erefrom during the processing thereof.

In the processing of monocalcium phosphate liquors the bone black treatment is preferably carried out during the usual lime reaction with the crude phosphoric acid after the addition of lime to the point where the liquor has a pH within the range of about 2.7 to 2.9. Free P205 and pH value are influenced by the temperature at which the lime reaction is carried out. With increased temperature, the free P205 increases with concurrent lowering of pH value and precipitation of excessive quantity of dicalcium phosphate as the monocalcium phosphate end point is approached. Liming is accompanied by precipitation of iron and aluminum phosphates together with some dicalcium phosphate and calcium-fluorine compounds, i. e. white mud; the presence of this white mud, I have found, aids in fluorine removal by the bone black treatment in accordance with this invention. Use of lime in amount in excess of the stoichiometric amount necessary to combine with all of the phosphoric acid'present to produce monocalcium phosphate will not satisfactorily remove the fluorine and will result in formation of large amounts of dicalcium phosphate. Increase of temperature, per se, does not interfere with fluorine removal from phosphate liquors by bone black.

Preferably, the production or monocalcium phosphate is carried out by treating the crude phosphoric acid, previously treated to -remove sulfate by well known methods, with lime in two steps; in the first step, about 98% of the total lime used is added, and bone black, in powdered or granular form, is mixed with the batch, which is then thoroughly agitated and filtered. The amount of bone black thus added may be from about 0.05 to about 0.1%, based on the weight of the phosphate present. The

remainder of the lime is added to the filtrate and from about 0.2 to about 0.3% of pone black is then added thereto and the mixture agitated. Thereafter the mixture may be gassed with hydrogen sulfide to effect the removal of lead and arsenic impurities, as sulfides, and the mixture filtered. Monocalcium phosphate may be crystallized from the filtrate.

The following example is illustrative of a preferred embodiment of the process of this invention:

4045 pounds of crude phosphoric acid containing 43% phosphoric acid expressed as P205 and .0035 fluorine, previously treated to remove sulfate, were treated with an aqueous slurry containing 783 pounds of hydrated lime (which constitutes 98% of 799 pounds of lime, the total lime used in the liming of the phosphoric acid), producing a monocalcium phosphate batch constituted of 20,000 pounds of slurry containing .0015% fluorine. To this slurry were added 17 pounds of granular boneblack, the batch thoroughly' agitated for 30 minutes and then filtered. The resulting monocalcium phosphate filtrate contained .00065% fluorine. An aqueous slurry containing 16 pounds of hydrated lime (constituting the remaining 2% of the lime used) were added to the filtrate of 18,667 pounds and then pounds of bone black and the resultant mixture agitated for 1 hour, then gassed with h drogen sulfide and filtered, producing a filtra e containing .000l5% fluorine. The monocalcium understood that the invention is not limited to the treatment of monocalcium phosphate liquors containing fluorine, but includes the processing of other phosphate liquors such as sodium phosphate liquors, dicalcium phosphate liquors, and others containing fluorine.

The monocalcium product produced in accordance with this invention has been found to contain an amount of fluorine substantially below the limit of .00l%; samples analyzed contained from .0004% to .0006% fluorine. Further, the product produced in accordance with this invention was found to be of an improved color as compared with products made under like conditions but not involving the bone black treatment of this invention. It was found that the use of bone black not only reduced the fluorine content materially, as above pointed out, but permitted the elimination of treatment of monocalcium phosphate liquors with Norite (activated carbon) which treatment has been resorted to in order to improve the color of the liquor. Moreover, the bone black aided in effecting the filtration of the monocalcium phosphate liquors from the precipitated materials and reduced the time required for effecting filtration.

In the specification and claims the term fluorine is used to refer not only to simple fluorides, butv also to complex compounds containing fluorine such as fiuosilicates.

It is to be understood that this invention is not restricted to the present disclosure otherwise than as defined by the appended claims.

What is claimed is:

1. A process for removing fluorine from a phosphate liquor having a pH of from about 1.5 to 3.5 and .containing not more than about .03% fluorine, which comprises adding bone black to the liquor, agitating the resulting mixture for a time sufflcient for the bone black to combine with the fluorine so as to reduce the fluorine content to a value such that a phosphate product crystallized therefrom has a fluorine content not exceeding about .001%, and then separating the purified liquor from the bone black.

2. A process of removing fluorine from monocalcium phosphate liquor having a pH of from about 1.5 to 3.5 and containing not more than about .03% fluorine, which comprises adding bone black to the liquor, agitating the resulting mixture for a suflicient time for the bone black to combine with the fluorine so as to reduce the fluorin content to a value such that the monocalcium phosphate crystallized therefrom has a fluorine content not exceeding about .00l% and Ellen separating the purified liquor from the bone ack.

3. In a process for purifying phosphate liquors containing fluorine to produce liquors from which food-grade products may be recovered, the steps which comprise contacting a fluorinecontaining phosphate liquor having a pH of from about 1.5 to 3.5 and containing not'more than about .03% fluorine with bone black for a time sufficient to cause the bone black to combine with the fluorine so as to produce a phosphate liquor having a fluorine content such that food-grade phosphate products may be recovered therefrom, ghd lseparating the purified liquor from the bone 4. The method of purifying calcium phosphate liquor containing not more than about .03% fluorine and having a pH of from about 1.5 to about 3.5, produced by the treatment of crude phosphoric acid with lime, which comprises adding about .1% to 1% bone black to the liquor after the addition of lime to the crude acid in amount suflicient to produce a liquor having a pH of from about 1.5 to about 3.5, agitating the mixture for a time sufficient for the bone black to combine with the fluorine so as to reduce the fluorine content to a value such that calcium phosphate crystallized therefrom has a fluorine content not exceeding about .00l%, and then separating the calcium phosphate liquor from the bone black and precipitated impurities.

5. The process of preparing monocalcium phosphate which comprises reacting lime with crude phosphoric acid to form a monocalcium phosphate solution having a pH of from about 1.5 to about 3.5 and fluorine not exceeding about .03%, adding from about .15 to about 3% finely divided bone black to this solution, agitating the mixture for a time sufflcient for the bone black to combine with the fluorine so as to reduce the fluorine content to a value such that monocalcium phosphate crystallized therefrom has a fluorine content not exceeding about .001%, and then separating the purified solution from the bone black and impurities removed from the solution.

6. The process of preparing monocalcium phosphate from crude phosphoric acid which comprises (1) treating crude phosphoric acid with lime to form a monocalcium phosphate solution containing not more than about'.03% fluorine, (2) adding-bone black to the lime-treated crude phosphoric acid and agitating the mixture for a time suflicient for the bone black to combine with the fluorine, (3) separating the monocalcium phosphate solution from the bone black and white mud produced in step 1, (4) treating the. monocalcium phosphate solution thus separated with additional lime, (5) adding a further amount of bone black to the lime-treated solution of step 4 and agitating the mixture for a time bufilcient for the bone black to combine with'the fluorine so as to reduce the fluorine content to a value such that monocalcium phosphate crystallized therefrom has a fluorine content not exceeding about .001%, and (6) separating the purified monocalcium phosphate solution from the bone black.

7. The process of preparing monocalcium phosphate from crude phosphoric acid which comprises (1) treating the crude phosphoric acid with lime to produce a monocalcium phosphate solution having a pH of from about 2.7 to about 2.9 and containing not more than about .03% fluorine, (2) adding from about .05% to about .l% bone black to the lime-treated crude phosphoric acid and agitating the mixture for a time sufficient for the bone black to combine with the fluorine, (3) separating the monocalcium phosphate solution from the bone black and White mu produced in step 1, ('4) treating the monocalcium phosphate solution from step 3 with additional lime, (5) adding from about .2% to about .3% bone black to the lime-treated solution of step 4 and agitating the mixture for a time sufficient for the bone black to combine with the fluorine so as to reduce the fluorine content to a value such that monocalcium phosphate crystallized therefrom has a fluorine content not exceeding about .001%, and (6) separating the purified monocalcium phosphate solution from the bone black.

. SIHON C. OGBURN, JR. 

